Process for producing polyolefins and polyolefin catalyst

ABSTRACT

The present invention provides a process for preparing polyolefins having a multimodal or at least bimodal molecular weight distribution by contacting in a reaction mixture under polymerization conditions at least one olefin and a catalyst system comprising a supported catalyst-component comprising an alumoxane and at least two metallocenes containing the same transition metal and selected from mono, di, and tri-cyclopentadienyls and substituted cyclopentadienyls of a transition metal and wherein at least one of the metallocenes is bridged and at least one of the metallocenes is unbridged.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method for preparing polyolefins having abi- or multimodal molecular weight distribution. This invention alsorelates to a polyolefin polymerization catalyst system. This inventionfurther relates to a method for preparing an olefin polymerizationcatalyst system.

BACKGROUND OF THE INVENTION

Polyolefins having a multimodal molecular weight distribution (MWD) canbe converted into articles by extrusion molding, thermoforming,rotational molding, etc. and have advantages over typical polyolefinslacking the multimodal MWD. Polyolefins having a multimodal MWD may beprocessed more easily, i.e. they can be processed at a faster throughputrate with lower energy requirements and at the same time such polymersevidence reduced melt flow perturbations and are preferred due toimproved properties for applications such as high strength films.

There are several known methods for producing polyolefins having amultimodal MWD; however, each method has its own disadvantages.Polyolefins having a multimodal MWD can be made by employing twodistinct and separate catalysts in the same reactor each producing apolyolefin having a different MWD; however, catalyst feed rate isdifficult to control and the polymer particles produced are not uniformin size, thus, segregation of the polymer during storage and transfercan produce non-homogeneous products. A polyolefin having a bimodal MWDcan also be made by sequential polymerization in two separate reactorsor by blending polymers of different MWD during processing; however,both of these methods increase capital cost.

European Patent No. 0128045 discloses a method of producing polyethylenehaving a broad molecular weight distribution and/or a multimodal MWD.The polyethylenes are obtained directly from a single polymerizationprocess in the presence of a catalyst system comprising two or moremetallocenes each having different propagation and termination rateconstants, and aluminoxane.

There are certain limits to the known methods for preparing bimodalmolecular weight distribution or multimodal molecular weightdistribution polyolefins. Even under ideal conditions the gel permeationchromatograph curves don't show a marked bimodal MWD of the polyolefin.The MWD and shear rate ratios of the polymer and the catalyst activitydisclosed in the known methods are rather poor. Further the knownmetallocene catalyst systems for producing bimodal MWD use aluminoxaneas cocatalyst during the polymerization which causes severe foulinginside the reactor and renders the use of such a type of catalyst incontinuous processes almost impossible.

It is therefore not surprising that none of the known methods forproducing a multimodal MWD polyolefin from a single polymerizationprocess in the presence of a catalytic system comprising at least twometallocenes have been developed at an industrial scale.

It is an object of the present invention to provide for a new processfor preparing polyolefins having a multimodal molecular weightdistribution. It is an object of the present invention to provide a newhigh activity polymerization catalyst system. It is a further object ofthe present invention to provide for a new process for preparing thepolymerization catalyst system of the present invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, polyolefins having amultimodal or at least bimodal molecular weight distribution areprepared by contacting in a reaction mixture under polymerizationconditions at least one olefin, a catalyst system comprising (a) asupported catalyst-component comprising an alumoxane and at least twometallocenes containing the same transition metal and selected frommono, di, and tri-cyclopentadienyls and substituted cyclopentadienyls ofa transition metal wherein at least one of the metallocenes is bridgedand at least one of the metallocenes is unbridged and (b) a cocatalyst.

While alumoxane can be used as a cocatlyst, the Applicant has found thatit was not necessary to use alumoxane as cocatalyst during thepolymerization procedure for preparing polyolefins according to theprocess of the present invention. Further the use of alumoxane as acocatalyst during the polymerization may lead to the fouling of thereactor.

According to a preferred embodiment of the present invention, one ormore cocatalysts represented by the formula MR_(x) are used, wherein Mis a metal selected from Al, B, Zn, Li and Mg, each R is the same ordifferent and is selected from halides or from alkoxy or alkyl groupshaving from 1 to 12 carbon atoms and x is from 1 to 3. Especiallysuitable cocatalysts are trialkylaluminium selected fromtrimethylaluminium, triethylaluminium, triisobutylaluminium,tri-n-hexylaluminium or tri-n-octylaluminium, the most preferred beingtriisobutylaluminium.

In accordance with the present invention the broadness of the molecularweight distribution and the average molecular weights can be controlledby selecting the catalyst system. In a preferred embodiment of thepresent-invention, this control is also performed by the introduction ofsome amount of hydrogen during polymerization. Another preferredembodiment of the present invention implies the use of a comonomer forthis control; examples of comonomer which can be used include 1-olefinssuch as 1-butene, 1-hexene, 1-octene, 4-methyl-pentene, and the like,the most preferred being 1-hexene.

It has unexpectedly been found that the polymerization process can beconducted under slurry phase polymerization conditions and thisconstitutes a real advantage of the process of the present invention.While slurry phase polymerization may be conducted under well knownoperating conditions, it is preferred that it is operated at atemperature of about 20° to 125° C. and a pressure of about 0.1 to 5.6MPa for a time between 10 minutes and 4 hours.

Another advantage of the present invention is that a continuous reactorcan be used for conducting the polymerization. This continuous reactoris preferably a loop reactor. During the polymerization process, theolefin monomer(s), the catalytic system, the cocatalyst and a diluentare flowed in admixture through the reactor.

A further advantage of the present invention is that the bulk density ofthe polymer obtained by the process of the present invention isparticularly high. The bulk density is an important characteristic ofthe polymer. The bulk density, commonly expressed in terms of grams percubic centimeters, should be relatively high. If the bulk density is toolow, the polymer will tend to be fluffy and will tend to cause pluggingand handling problems in the product transfer system. Low bulk densitiesmean problems for fluff packaging and for the extrusion processing. Thisis particularly important in a continuous or a semi-continuouspolymerization where plugging of the withdrawal outlet or another pointin the polymerization system can cause serious interruptions inproduction schedules.

According to the present invention when hydrogen is used it is preferredthat the relative amounts of hydrogen and olefin introduced into thepolymerization reactor be within the range of about 0.001 to 15 molepercent hydrogen and 99.999 to 85 mole percent olefin based on totalhydrogen and olefins present, preferably about 0.2 to 3 mole percenthydrogen and 99.8 to 97 mole percent olefin.

It is preferred that the polymerization reaction be run in a diluent ata temperature at which the polymer remains as a suspended solid in thediluent. Diluents include, for examples, isobutane, n-hexane, n-heptane,methylcyclohexane, n-pentane, n-butane, n-decane, cyclohexane and thelike. The preferred diluent is isobutane.

The olefin monomer used in the process of the present invention toproduce a polyolefin of bimodal or multimodal molecular weightdistribution in which each polymer particle contains both high and lowmolecular weight polymer molecules is preferably selected from ethyleneand mono-1-olefins (alpha olefins), preferably mono-1-olefins havingfrom 2 to 10 carbon atoms including for example, 4-methyl-1-pentene.More preferably these mono-1-olefins are selected from the groupconsisting of ethylene, propylene, and mixtures thereof; ethylene beingthe most preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing one embodiment of the presentinvention.

FIGS. 2 through 20 are graphs showing the corresponding results forExamples 5 through 23, respectively, set forth in Table 2.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, the supported catalyst-componentused in the process for producing polyolefins having multimodalmolecular weight distribution can be made by any known method as long asit comprises an alumoxane and at least two metallocenes containing thesame transition metal wherein at least one of the metallocenes isbridged and at least one of the metallocenes is unbridged.

Known processes of producing these types of catalysts are disclosed inEuropean Patent No. 0206794, the content of which is incorporated byreference. This patent discloses a catalyst-component comprising thereaction product of at least one metallocene and alumoxane in thepresence of a support material thereby providing a supportedmetallocene-alumoxane reaction product as the sole catalyst component.

The metallocenes used in the process of the present invention areorganometallic coordination compounds which are cyclopentadienylderivatives of a Group 4b, 5b or 6b metal of the Periodic Table andinclude mono, di and tricyclopentadienyls and their derivatives of thetransition metals. Particularly desirable are the metallocene of a Group4b and 5b metal such as titanium, zirconium, hafnium and vanadium.

The preferred metallocenes can be represented by the general formulae:

    I. (Cp).sub.m MR.sub.n X.sub.q

wherein Cp is a cyclopentadienyl ring, M is a Group 4b, 5b, or 6btransition metal, R is a hydrocarbyl group or hydrocarboxy having from 1to 20 carbon atoms, X is a halogen, and m=1-3, n=0-3, q=0-3 and the sumof m+n+q will be equal to the oxidation state of the metal.

    II. (C.sub.5 R'.sub.k).sub.g R".sub.s (C.sub.5 R'.sub.k)MQ.sub.3-g

and

    III. R".sub.s (C.sub.5 R'.sub.k).sub.2 MQ'

wherein (C₅ R'_(k)) is a cyclopentadienyl or substitutedcyclopentadienyl, each R' is the same or different and is hydrogen orhydrocarbyl radical such as alkyl, alkenyl, aryl, alkylaryl, orarylalkyl radical containing from 1 to 20 carbon atoms or two carbonatoms are joined together to form a C₄ -C₆ ring, R" is a C₁ -C₄ alkyleneradical, a dialkyl germanium or silicon or siloxane, or an alkylphosphine or amine radical bridging two (C₅ R'_(k)) rings, Q is ahydrocarbyl radical such as aryl, alkyl, alkenyl, alkylaryl, or arylalkyl radical having from 1-20 carbon atoms, hydrocarboxy radical having1-20 carbon atoms or halogen and can be the same or different from eachother, Q' is an alkylidiene radical having from 1 to about 20 carbonatoms, s is 0 or 1, g is 0, 1 or 2, s is 0 when g is 0, k is 4 when s is1 and k is 5 when s is 0, and M is as defined above. Exemplaryhydrocarbyl radicals are methyl, ethyl, propyl, butyl, amyl, isoamyl,hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, 2-ethylhexyl,phenyl and the like. Exemplary halogen atoms include chlorine, bromine,fluorine and iodine and of these halogen atoms, chlorine is preferred.Exemplary hydrocarboxy radicals are methoxy, ethoxy, propoxy, butoxy,amyloxy and the like. Exemplary of the alkylidiene radicals ismethylidene, ethylidene and propylidene.

According to a preferred embodiment of the present invention, thecatalyst-component comprises at least two metallocenes deposited on asupport wherein:

At least one of the metallocenes is unbridged and is represented by theformula (Cp)₂ MX₂ wherein each Cp is the same or different and isselected from substituted or unsubstituted cyclopentadienyl, indenyl orfluorenyl, M is zirconium, titanium or hafnium and X, which is the sameor different, is a hydrocarbyl radical such as aryl, alkyl, alkenyl,alkylaryl, or aryl alkyl radical having from 1-20 carbon atoms or ahalogen.

At least one of the metallocenes is bridged and is represented by theformula R"(Cp)₂ MX₂ wherein each Cp is the same or different and isselected from substituted or unsubstituted cyclopentadienyl, indenyl orfluorenyl, M is zirconium, titanium or hafnium, X, which is the same ordifferent, is a hydrocarbyl radical such as aryl, alkyl, alkenyl,alkylaryl, or aryl alkyl radical having from 1-20 carbon atoms or ahalogen and R" is a C₁ -C₄ alkylene radical, a dialkyl germanium orsilicon or siloxane, or an alkyl phosphine or amine radical bridging two(Cp) rings.

Preferably, in the above-identified formulae, for the unbridgedmetallocene Cp is a substituted or unsubstituted cyclopentadienyl orindenyl, M is zirconium, titanium or hafnium and X is Cl or CH₃, and forthe bridged metallocene Cp is a substituted or unsubstitutedcyclopentadienyl, indenyl or fluorenyl, M is zirconium, titanium orhafnium, X is Cl or CH₃ and R" is an ethylene radical or silicon.

Preferably, the unbridged metallocene is a bis(cyclopentadienyl)zirconium dichloride and the bridged metallocene is anethylene-bis(indenyl) zirconium dichloride.

The molar ratio of the unbridged metallocenes to the bridgedmetallocenes can vary over a wide range, and in accordance with thepresent invention, the only limitation on the molar ratio is the breadthof the molecular weight distribution (MWD) and the degree of bimodalitydesired in the product polymer. Preferably, the unbridged to bridgedmetallocenes molar ratio will be between 10:1 and 1:10, preferablybetween 5:1 and 1:5, more preferably between 4:1 and 2:1.

The alumoxanes used in the process of the present invention are wellknown and preferably comprise oligomeric linear and/or cyclic alkylalumoxanes represented by the formula:

    (I) ##STR1## for oligomeric, linear alumoxanes and

    (II) ##STR2## for oligomeric, cyclic alumoxane,

wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R isa C₁ -C₈ alkyl group and preferably methyl. Generally, in thepreparation of alumoxanes from, for example, aluminum trimethyl andwater, a mixture of linear and cyclic compounds is obtained.

The support used in the process of the present invention can be any ofthe solid, particularly, porous supports such as talc, inorganic oxides,and resinous support materials such as polyolefin. Preferably, thesupport material is an inorganic oxide in its finely divided form.Suitable inorganic oxide materials which are desirably employed inaccordance with this invention include Group 2a, 3a, 4a or 4b metaloxides such as silica, alumina and mixtures thereof. Other inorganicoxides that may be employed either alone or in combination with thesilica or alumina are magnesia, titania, zirconia, and the like. Othersuitable support materials, however, can be employed, for example,finely divided functionalized polyolefins such as finely dividedpolyethylene. Preferably, the support is a silica having a surface areacomprised between 200 and 600 m² /g and a pore volume comprised between0.5 and 3 ml/g.

The amount of alumoxane and metallocenes usefully employed in thepreparation of the solid support catalyst can vary over a wide range.Preferably the aluminum to transition metal mole ratio is comprisedbetween 1:1 and 100:1, preferably between 5:1 and 50:1.

The order of addition of the metallocenes and alumoxane to the supportmaterial can vary. In accordance with a preferred embodiment of thepresent invention alumoxane dissolved in a suitable inert hydrocarbonsolvent is added to the support material slurried in the same or othersuitable hydrocarbon liquid and thereafter a mixture of the at least twometallocenes is added to the slurry.

According to a preferred embodiment of the present invention, thesupported component catalyst is prepared by mixing together theunbridged metallocene alumoxane supported catalyst with the bridgedmetallocene alumoxane supported catalyst.

Preferred solvents include mineral oils and the various hydrocarbonswhich are liquid at reaction temperatures and which do not react withthe individual ingredients. Illustrative examples of the useful solventsinclude the alkanes such as pentane, iso-pentane, hexane, heptane,octane and nonane; cycloalkanes such as cyclopentane and cyclohexane,and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.Preferably the support material is slurried in toluene and themetallocene and alumoxane are dissolved in toluene prior to addition tothe support material.

The following examples are illustrative of the claimed invention andshould not be interpretive as limiting the scope thereof.

EXAMPLES

1. Catalyst preparation (A)

The support used is a silica having a surface area of 322 m² /g (GRACE952). This silica is further prepared by drying in high vacuum on aschlenk line for three hours to remove the physically absorbed water andthen suspended in toluene to react with methyl alumoxane (MAO) for threehours at the reflux temperature. Finally it is cooled and washed threetimes with toluene to remove the unreacted MAO. A solution of the twocorresponding metallocenes, (Cp) ₂ ZrCl₂ and ethylene (Ind) ₂ ZrCl₂ intoluene is added to the treated silica and the mixture is stirred for anhour. The supernatant liquid was filtered off and the remaining solidwas dried under vacuum after being washed three times with toluene.Three minutes before the introduction of the catalyst into the reactionzone 1 ml of 25 wt % of triisobutylaluminium (TIBAL) in toluene isadded. All polymerizations were performed in a two liter Buchi reactorin one liter of iso-butane as diluent.

2. Polymerization procedure (A)

A suspension of supported catalyst is introduced into the reactor underthe iso-butane pressure. The polymerization is initiated by pressurizingthe reactor with 30 bars of ethylene. The ethylene pressure ismaintained during the whole duration of the polymerization. Thepolymerization is stopped by cooling the reactor and venting theethylene. The polymer is recovered and analyzed. The catalyst type, thepolymerization conditions and the polymer properties are given in Table1.

3. Catalyst preparation (B)

The two supports used are MAO supported silica identical to the oneprepared in method (A) hereabove.

(a) a solution of (Cp)₂ ZrCl₂ in toluene is deposited on the firstsupport by stirring the resulting suspension for one hour at ambienttemperature. The supernatant liquid was filtered off and the remainingsolid was dried under vacuum after being washed three times withtoluene.

(b) a solution of ethylene (Ind)₂ ZrCl₂ in toluene is deposited on thesecond support by stirring the resulting suspension for one hour atambient temperature. The supernatant liquid was filtered off and theremaining solid was dried under vacuum after being washed three timeswith toluene.

(c) the two separately obtained supported metallocenes (a) and (b) weremixed together in a 2:1 weight ratio ((a):(b)).

4. Polymerization procedure (B)

The reactor used in all examples has a capacity of 35 liters and iscontinuously agitated. This continuous reactor is first filled withisobutane at a pressure of 40 bars. Then, as indicated in FIG. 1, asuspension of supported catalyst (1), isobutane (2), TIBAL (3), hexene(4), ethylene (5) and hydrogen (6) are continuously introduced into thereactor. The polymers are recovered at (9). All polymers were analyzedby Gel Permeation Chromatography (GPC-WATERS MILLIPORE) and DifferentialScanning Calorimetry (DSC). The graphs are given in FIGS. 2 to 20 (FIGS.2 to 20 respectively correspond to examples 5 to 23 of Table 2). "D"represents the ratio Mw/Mn (MWD), "D'" the ratio Mz/Mw and "A" the areaunder the curve. The polymerization conditions and the polymerproperties are given in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Catalyst   Hexene                                                                            Hydrogen                                                                           Yield                                                                            Activity                                                                          Bulk (1)                                                                          MI.sub.2 (2)                                                                      HLMI (3)                                   Example                                                                            (mg)                                                                             type                                                                             (ml)                                                                              (Nl) (g)                                                                              (g/g · h)                                                                (g/cc)                                                                            (g/10')                                                                           (g/10')                                                                            SRR (4)                               __________________________________________________________________________    1    52 (B)                                                                              10  4.5  100                                                                              1025                                                                              0.3 (a) (a)  (b)                                        99 (A)                                                                   2    50 (B)                                                                              10  1    135                                                                              1126                                                                              0.27                                                                              1.15                                                                              49.5 43.04                                      100                                                                              (A)                                                                   .sup.   3(x)                                                                       50 (B)                                                                              10  2.5  120                                                                               946                                                                              0.3 0.61                                                                              23.9 39.18                                      100                                                                              (A)                                                                   4    50 (B)                                                                               5  1    170                                                                               662                                                                              0.25                                                                              0.58                                                                              22.3 38.44                                      100                                                                              (A)                                                                   __________________________________________________________________________     (A) bis(cyclopentadienyl) zirconium dichloride                                (B) ethylenebis(indenyl) zirconium dichloride                                 (x) precontact time of 45 minutes between catalyst and cocatalyst before      polymerization                                                                (1) Bulk Density (ASTMD-1895)                                                 (2) Melt Index (ASTMD-1238-89A)                                               (3) High Load Melt Index (ASTMD-1238-89A)                                     (4) Shear Rate Response (HLMI/MI.sub.2)                                       (a) too high to be measured                                                   (b) non determined                                                           Catalyst   Density (1)                                                                         Mz  Mw    Mn        MP (2)                                                                            H (3)                                Example                                                                            (mg)                                                                             type                                                                             (g/cc)                                                                              x10E3                                                                             x10E3 x10E3                                                                             MWD   (°C.)                                                                      (J/g)                                __________________________________________________________________________    1    52 (B)                                                                              (b)   (b) (b)   (b) (b)   127.8                                                                             192                                       99 (A)                                                                   2    50 (B)                                                                              0.9521                                                                              (b) (b)   (b) (b)   131 180                                       100                                                                              (A)                                                                   .sup.   3(x)                                                                       50 (B)                                                                              0.9518                                                                              1100                                                                              137   12  11    132.3                                                                             189.5                                     100                                                                              (A)                                                                   4    50 (B)                                                                              0.9408                                                                              1230                                                                              132   15.5                                                                              8.5   133 191.2                                     100                                                                              (A)                                                                   __________________________________________________________________________     (A) bis(cyclopentadienyl) zirconium dichloride                                (B) ethylenebis(indenyl) zirconium dichloride                                 (x) precontact time of 45 minutes between catalyst and cocatalyst before      polymerization                                                                (1) ASTMD-1505-85                                                             (2) Melting Point (DSC)                                                       (3) Enthalpy of fusion (DSC)                                                  (b) non determined                                                       

                                      TABLE 2                                     __________________________________________________________________________      Cata TIBAL                                                                             i-C.sub.4                                                                         C.sub.2                                                                           C.sub.6                                                                           H.sub.2                                                                           Bulk (1)                                                                          MI (2)                                                                            HLMI (3) D (5)                                                                            Mz  Mw  Mn                     Ex                                                                              (g/h)                                                                              (g/h)                                                                             (kg/h)                                                                            (kg/h)                                                                            (cc/h)                                                                            (Nl/h)                                                                            (g/cc)                                                                            (g/10')                                                                           (g/10')                                                                            SRR (4)                                                                           (g/cc)                                                                           x10E3                                                                             x10E3                                                                             x10E3                                                                             MWD                __________________________________________________________________________     5                                                                              4.5  7.2 18  3   0   33  0.38                                                                              0.22                                                                              45.9 204 0.965                                                                            2218                                                                              254 7.7 33.0                6                                                                              4.5  7.2 18  2.5 0   14  0.37                                                                              0.12                                                                              30.6 251 0.963                                                                            2492                                                                              300 9.0 33.3                7                                                                              4.5  7.2 18  2.5 0   15  0.37                                                                              0.05                                                                              13.9 259 0.965                                                                            2699                                                                              357 10.2                                                                              35.0                8                                                                              4.5  7.2 18  2.5 0   15  0.35                                                                              0.05                                                                              13.6 300 0.964                                                                            2868                                                                              381 10.2                                                                              37.4                9                                                                              4.5  7.2 16  2   268 15  0.38                                                                              0.07                                                                              15.7 225 0.958                                                                            2644                                                                              337 9.4 35.7               10                                                                              4.5  7.2 16  2   265 15  0.39                                                                              0.09                                                                              23.9 266 0.959                                                                            2486                                                                              323 9.4 34.5               11                                                                              4.5  7.2 16  2   270 15  0.39                                                                              0.08                                                                              24.0 312 0.958                                                                            2390                                                                              307 9.0 34.1               12                                                                              4.5  7.2 16  2   274 15  0.39                                                                              0.11                                                                              24.8 232 0.958                                                                            2402                                                                              308 9.1 33.8               13                                                                              4.5  7.2 16  2   268 15  0.39                                                                              0.10                                                                              24.5 255 0.959                                                                            2437                                                                              325 9.7 33.6               __________________________________________________________________________     Catalyst preparation (B)                                                      (1) Bulk Density (ASTMD-1895)                                                 (2) Melt Index (ASTMD-1238-89A)                                               (3) High Load Melt Index (ASTMD-1238-89A)                                     (4) Shear Rate Response (HLMI/MI.sub.2)                                       (5) Density (ASTMD-1505-85)                                                   TIBAL triisobutylaluminium, iC.sub.4 isobutane, C.sub.2 ethylene, C.sub.6     hexene                                                                   

      Cata TIBAL                                                                             i-C.sub.4                                                                         C.sub.2                                                                           C.sub.6                                                                           H.sub.2                                                                           Bulk (1)                                                                          MI.sub.2 (2)                                                                      HLMI (3) D (5)                                                                            Mz  Mw  Mn                     Ex                                                                              (g/h)                                                                              (g/h)                                                                             (kg/h)                                                                            (kg/h)                                                                            (cc/h)                                                                            (Nl/h)                                                                            (g/cc)                                                                            (g/10')                                                                           (g/10')                                                                            SRR (4)                                                                           (g/cc)                                                                           x10E3                                                                             x10E3                                                                             x10E3                                                                             MWD                __________________________________________________________________________    14                                                                              4.5  7.2 18  2   0   0   0.30                                                                              0.16                                                                              11.8 74  0.951                                                                            2068                                                                              216 19.3                                                                              11.2               15                                                                              4.5  7.2 18  2   0   0   0.28                                                                              0.08                                                                              6.5  80  0.951                                                                            2569                                                                              264 26.3                                                                              10.0               16                                                                              4.5  7.2 18  2   0   0   0.28                                                                              0.14                                                                              9.6  69  0.951                                                                            1751                                                                              198 18.7                                                                              10.6               17                                                                              4.5  7.2 18  2.5 198 0   0.30                                                                              0.09                                                                              7.4  80  0.945                                                                            2419                                                                              270 27.0                                                                              10.0               18                                                                              4.5  7.2 18  2.5 200 0   0.30                                                                              0.07                                                                              6.1  94  0.941                                                                            2498                                                                              282 29.0                                                                              9.7                19                                                                              4.5  7.2 18  2.5 205 0   0.30                                                                              0.05                                                                              5.6  110 0.941                                                                            2488                                                                              288 30.1                                                                              9.6                20                                                                              4.5  7.2 18  2.5 207 0   0.30                                                                              0.05                                                                              5.1  97  0.939                                                                            2257                                                                              270 32.0                                                                              8.4                21                                                                              4.5  7.2 16  2   202 10  0.30                                                                              0.13                                                                              11.4 87  0.945                                                                            2341                                                                              250 18.3                                                                              13.7               22                                                                              4.5  7.2 16  2   210 10  0.30                                                                              0.11                                                                              9.1  78  0.946                                                                            2653                                                                              261 19.0                                                                              13.8               23                                                                              4.5  7.2 16  2   200 12  0.29                                                                              0.10                                                                              7.9  78  0.942                                                                            2162                                                                              247 23.6                                                                              10.5               __________________________________________________________________________     Catalyst preparation (B)                                                      (1) Bulk Density (ASTMD-1895)                                                 (2) Melt Index (ASTMD-1238-89A)                                               (3) High Load Melt Index (ASTMD-1238-89A)                                     (4) Shear Rate Ratio (HLMI/MI.sub.2)                                          (5) Density (ASTMD-1505-85)                                                   TIBAL triisobutylaluminium, iC.sub.4 isobutane, C.sub.2 ethylene, C.sub.6     hexene                                                                   

We claim:
 1. A process for the preparation of polyolefins having amultimodal or at least bimodal molecular weight distributioncomprising:(1) providing a catalyst system comprising (a) a supportedcatalyst-component comprising an alumoxane and at least two supportedmetallocenes effective for olefin polymerization containing the sametransition metal and selected from the group consisting of mono, di- andtri-cyclopentadienyls and substituted cyclopentadienyls of a group 4b,5b or 6b transition metal wherein at least one of the supportedmetallocenes is bridged and at least one of the supported metallocenesis unbridged with the molar ratio of said unbridged to bridgedmetallocenes in said catalyst system being within the range of 4:1 and2:1 and (b) a cocatalyst; (2) contacting said catalyst system in apolymerization reaction zone with at least one olefin and maintainingsaid reaction zone under polymerization conditions to produce multimodalor at least bimodal molecular weight distribution polymer of saidolefin.
 2. The process according to claim 1 wherein the catalyst systemcomprises one or more cocatalyst represented by the formula MR_(x)wherein M is a metal selected from the group consisting of Al, B, Zn, Liand Mg, each R is the same or different and is selected from the groupconsisting of halides, and alkoxy or alkyl groups having from 1 to 12carbon atoms and x is from 1 to
 3. 3. The process according to claim 2wherein the cocatalyst is a trialkylaluminium selected from the groupconsisting of trimethylaluminium, triethylaluminium,triisobutylaluminium, tri-n-hexylaluminium and tri-n-octylaluminium. 4.The process of claim 3, wherein said metallocenes contain a transitionmetal selected from the group consisting of titanium, zirconium, hafniumand vanadium.
 5. The process according to claim 1 wherein thepolymerization reaction is run in a diluent selected from the groupconsisting of isobutane, n-hexane, n-heptane, methylcyclohexane,n-pentane, n-butane, n-decane, and cyclohexane.
 6. The process accordingto claims 1 wherein hydrogen is introduced into the polymerizationreaction zone in an amount ranging from about 0.001 to 15 mole percenthydrogen based on total hydrogen and olefin present.
 7. The processaccording to claims 1 whereinthe unbridged metallocenes are representedby the formula (Cp)₂ MX₂ wherein each Cp is the same or different and isselected from the group consisting of substituted and unsubstitutedcyclopentadienyl, indenyl or fluorenyl, M is zirconium, titanium orhafnium and X, which is the same or different, is a hydrocarbyl radicalselected from the group consisting of aryl, alkyl, alkenyl, alkylaryl,and aryl alkyl radical having from 1-20 carbon atoms and a halogen; thebridged metallocenes are represented by the formula R"(Cp)₂ MX₂ whereineach Cp is the same or different and is selected from the groupconsisting of substituted and unsubstituted cyclopentadienyl, indenyland fluorenyl, M is zirconium, titanium or hafnium and X, which is thesame or different, is a hydrocarbyl radical selected from the groupconsisting of aryl, alkyl, alkenyl, alkylaryl, and aryl alkyl radicalhaving from 1-20 carbon atoms or a halogen and R" is a C₁ -C₄ alkyleneradical, a dialkyl germanium or silicon or siloxane, or an alkylphosphine or amine radical bridging two (Cp) rings.
 8. The processaccording to claim 7 wherein, in the formula of the unbridgedmetallocene, Cp is a substituted or unsubstituted cyclopentadienyl orindenyl, M is zirconium, titanium or hafnium and X is Cl or CH₃, and inthe formula of the bridged metallocene Cp is a substituted orunsubstituted cyclopentadienyl, indenyl or fluorenyl, M is zirconium,titanium or hafnium, X is Cl or CH₃ and R" is an ethylene radical ordialkyl silicon.
 9. The process according to claim 8 wherein theunbridged metallocene is a bis(cyclopentadienyl) zirconium dichlorideand the bridged metallocene is an ethylene-bis(indenyl) zirconiumdichloride.
 10. The process of claim 1, wherein said supported catalystcomponent comprises said two metallocenes collectively supported on thesame support.
 11. The process of claim 10, wherein said supportedcatalyst component is prepared by adding a solution of the twometallocenes to the same support.
 12. The process of claim 11, whereinsaid alumoxane is added to said support prior to said metallocenes. 13.The process of claim 1, wherein said bridged metallocene is deposited ona first support and a unbridged metallocene is deposited on a secondsupport followed by mixing the two separately supported metallocenestogether.
 14. The process of claim 13, wherein said alumoxane is addedto said supports prior to the deposition of said metallocenes on saidsupports.
 15. In a process for the preparation of polyolefins having amultimodal or at least bimodal molecular weight distribution, the stepscomprising(1) providing a catalyst system comprising (a) a finelydivided support material containing an alumoxane and at least twometallocenes effective for olefin polymerization supported on saidsupport material, said metallocenes containing the same transition metaland selected from the group consisting of mono, di- andtri-cyclopentadienyls and substituted cyclopentadienyls of a group 4b,5b or 6b transition metal wherein at least one of the supportedmetallocenes is bridged and at least one of the supported metallocenesis unbridged and the molar ratio of said unbridged to bridgedmetallocenes in said catalyst system being within the range of 4:1 and2:1; (2) providing a cocatalyst; and (3) contacting said catalyst systemin a polymerization reaction zone with at least one olefin andmaintaining said reaction zone under polymerization conditions toproduce multimodal or at least bimodal molecular weight distributionpolymer of said olefin.
 16. The process of claim 15 wherein saidsupported catalyst component is prepared by adding a solution of the twometallocenes to said support material.
 17. In a process for thepreparation of polyolefins having a multimodal or at least bimodalmolecular weight distribution comprising the steps of:(1) providing acatalyst system comprising at least two supported metallocenes effectivefor olefin polymerization and containing the same transition metal andselected from the group consisting of mono, di- andtri-cyclopentadienyls and substituted cyclopentadienyls of a group 4b,5b or 6b transition metal wherein at least one of the supportedmetallocenes is bridged and at least one of the supported metallocenesis unbridged, said bridged metallocene being deposited on a first finelydivided support material and said unbridged metallocene being depositedon a second finely divided support material which is in admixture withsaid first support material; (2) providing a cocatalyst; and (3)contacting said catalyst system in a polymerization reaction zone withat least one olefin and maintaining said reaction zone underpolymerization conditions to produce multimodal or at least bimodalmolecular weight distribution polymer of said olefin.
 18. The process ofclaim 17 wherein the molar ratio of said unbridged to bridgedmetallocenes in said catalyst system is within the range of 4:1 and 2:1.19. In a process for the preparation of polyolefins having a multimodalor at least bimodal molecular weight distribution, the stepscomprising:(1) providing a finely divided support material containing analumoxane; (2) contacting said support material with a mixture of atleast two metallocenes effective for olefin polymerization containingthe same transition metal and selected from the group consisting ofmono, di- and tri-cyclopentadienyls and substituted cyclopentadienyls ofa group 4b, 5b or 6b transition metal wherein at least one of themetallocenes is bridged and at least one of the metallocenes isunbridged to produce a catalyst system; (3) providing a cocatalyst; and(4) contacting said catalyst system and cocatalyst in a polymerizationreaction zone with at least one olefin and maintaining said reactionzone under polymerization conditions to produce multimodal or at leastbimodal molecular weight distribution polymer of said olefin.
 20. In aprocess for the preparation of polyolefins having a multimodal or atleast bimodal molecular weight distribution, the steps comprising:(1)providing a first finely divided support material containing analumoxane; (2) contacting said support material with a bridgedmetallocene effective for olefin polymerization selected from the groupconsisting of di- and tri-cyclopentadienyls and substitutedcyclopentadienyls of a group 4b, 5b or 6b transition metal; (3)providing a second finely divided support material containing analumoxane; (4) contacting said second support material with an unbridgedmetallocene effective for olefin polymerization containing the sametransition metal and selected from the group consisting of mono, di- andtri-cyclopentadienyls and substituted cyclopentadienyls of a group 4b,5b or 6b transition metal which is the same transition metal as in saidbridged metallocene; (5) mixing the products of steps 2 and 4 togetherto provide a catalyst system; (6) providing a cocatalyst; and (7)contacting said catalyst system and cocatalyst in a polymerizationreaction zone with at least one olefin and maintaining said reactionzone under polymerization conditions to produce multimodal or at leastbimodal molecular weight distribution polymer of said olefin.